Method for passivating a zinc surface

ABSTRACT

A method of passivating a metallic surface to improve the subsequent adhesion of paint thereto comprising directing a passivating liquid through an electrostatic field and onto the metallic surface, the electrostatic field atomising the passivating liquid.

United States Patent 1191 Costelloe Nov. 27, 1973 METHOD FOR PASSIVATING A ZINC [56] References Cited SURFACE UNITED STATES PATENTS [75] Inventor: Patrick Costelloe, Chester, England 2,826,451 3/1958 Sedlacsik 1l7/93.4 R 1731 Assignw British Steel Corporation, London, 31232158; 31133? 2222215.??? 111111173852 1? England 3,594,214 7/1971 Helwig et a1 117/107 2 Oct. 3,595,687 7/1971 Labana 117/9331 [21] Appl' 187,649 Primary Examiner-Ralph S. Kendall Assistant Examiner- M. G. Wityshyn [30] Foreign Application Priority Data Almmeycushman, Darby & Cushman Oct. 13, 1970 Great Britain 48,583/70 [57] ABSTRACT [52 us. c1. 148/6.2, 148/6.15, 118/634, A method of passivating a metallic smface to improve 117/934 117/119 the subsequent adhesion of paint thereto comprising [51] Int. C1. C231 9/02, B051) 5/02 directing a passivating liquid through an electrostatic [58] Field of Search 148/62, 6.3; field and onto the metallic surface the electrostatic ll7/93.4, 93.1 R, 93.31, 107, 119, 93.1 GD, 93.1 CD; 118/629, 630, 634, 50.1

field atomising the passivating liquid.

13 Claims, 1 Drawing Figure METHOD FOR PASSIVATKNG A ZINC SURFACE This invention-concerns a method and an apparatus for passivating a metallic surface to improve the subsequent adhesion of paint thereto, and, although the invention is not so restricted, it is more particularly concerned with the galvanising, passivating and painting of a ferrous substrate.

The painting of a galvanised ferrous substrate, as is well-known, affords substantial protection to the substrate additional to that provided by the zinc coating thereof, but necessitates the prior passivation of the zinc coating. This passivation is necessary because, if this is not done, the adhesion of the paint to the zinc coating is poor, while a chemical reaction which occurs between the zinc coating, the constituents of the paint, and moisture which penetrates the paint, can lead to blistering and flaking of the paint during service. The passivating treatment, moreover, which is also known as conversion coating, not only assists paint adhesion, but also prevents the final product from white rusting during storage.

According to the present invention, there is provided a method of passivating a metallic surface to improve the subsequent adhesion of paint thereto comprising directing a passivating liquid through an electrostatic field and onto the metallic surface, the electrostatic field atomising the passivating liquid.

It has been found that the passage of the passivating liquid through the electrostatic field increases its reactivity.

The passivating liquid is preferably so directed through a liquid outlet aperture of a container for the passivating liquid, the container being provided with an electrode which extends through said aperture, and a high intensity electric discharge being created between the said electrode and the metallic surface.

The passivating liquid may be directed through the said outlet aperture by subjecting the passivating liquid in the container to a gas pressure which exceeds that prevailing externally of the container.

The passivating liquid may be a phosphating solution, but is preferably a chromating solution such, for example, as that marketed under the Trade Name Accomet C.

The said metallic surface is preferably that of a zinc coating on a ferrous substrate.

A metal substrate may be provided with a metallic coating, which provides the said surface, while the substrate is disposed within a vacuum chamber, molten coating metal being evaporated under the reduced pressure in the vacuum chamber, and the vapour of the coating metal being condensed onto the substrate.

The metal substrate, moreover, is preferably a steel strip which is moved continuously through the vacuum chamber, the coating metal being zinc.

The passivating liquid is preferably directed onto the surface of the coating metal while the latter is in the vacuum chamber, the passivating liquid being an aqueous solution containing a low freezing point liquid whose freezing point is such that the passivating liquid is in the liquid state when it contacts the surface of the coating metal.

Since the said low freezing point liquid ensures that the passivating liquid is in the liquid state when it contacts the surface of the zinc coating metal, the water in the passivating liquid is able to perform its required chemical function in effecting passivation.

The passivating liquid preferably contains at least 20 percent, and may contain at least 30 percent, by volume of water.

The low freezing point liquid is preferably a glycol, e.g. diethylene glycol.

The pressure in the vacuum chamber preferably does not exceed 10 Torr and may, for example, not exceed 10 Torr.

The passivating liquid preferably reaches the surface of the coating metal in the form of droplets whose average diameter does not exceed 200 microns.

Preferably, while the substrate is disposed in the vacuum chamber, the passivated surface is first painted with a radiation curable, non solvent based paint, and the paint is then radiation cured.

The paint may be applied to the passivated surface by roller coating, and the paint so applied may be cured by the Videcolor electron bombardment process of the British Iron and Steel Research Association.

The invention also comprises a substrate when treated by the method set forth above.

Additionally, the invention comprises apparatus for use in the said method comprising a container for a passivating liquid, the said container having a liquid outlet aperture through which extends an electrode; means for supporting a metallic substrate adjacent to said electrode; means for creating a high intensity electric discharge between the electrode and the metallic substrate whereby to produce an electrostatic field in which the passivating liquid is atomised; and means for forcing the passivating liquid to pass through the outlet aperture and through the electrostatic field onto the metallic substrate.

The said apparatus may be disposed within a vacuum chamber, there also being provided within the said vacuum chamber means for coating the substrate with a metal prior to the application of the passivating liquid, and means for painting the passivated metallic coating.

The invention is illustrated, merely by way of example, in the accompanying drawing, which is a schematic view of an apparatus for carrying out the method of the present invention.

In the drawing, there is shown a vacuum chamber l the pressure within which may be of the order of 10* Torr. A mild steel strip 2, having opposite surfaces 3, 4, enters the vacuum chamber 1 through an air-tovacuum entry seal 5 and leaves the vacuum chamber 1 through a vacuum-to-air exit seal 6, the strip 2 being passed through the vacuumvchamber l at a speed of, for example, feet per minute. The strip 2, in passing from the entry seal .5 to the exit seal 6, passes over support rollers 7 to 12iinclusive which cause the strip 2 to pass successively through a first horizontal run in which the surface 3 of the strip is uppermost, a second horizontal run in which the surface 4 of the strip is uppermost, and a third horizontal run in which the surface 3 is again uppermost.

The surface 4 of the strip 2, while passing through the said first horizontal run, passes adjacent to and above a zinc applicator 14. Molten zinc in the zinc applicator 114 is evaporated under the reduced pressure in the vacuum chamber 1 and is condensed onto the surface 4 so as to provide the latter with a zinc coating (not shown).

Similarly, the surface 3 of the strip 2, whilst in the said second horizontal run, passes adjacent to and above a zinc applicator from which it receives a zinc coating (not shown).

The strip 2, prior to passing over the zinc applicator 15, passes beneath a passivating liquid applicator 16. As a result, the zinc coating on the surface 4, which will by this time have solidified, passes adjacent to and beneath the passivating liquid applicator 16.

The passivating liquid applicator 16 comprises a glass container having a tubular portion 20 which is adapted to receive an aqueous passivating solution. The latter may be constituted by a chromating solution containing 35 percent by volume of water, 60 percent by volume of diethylene glycol, and 5 percent by volume of Accomet C. The bottom of the tubular portion 20 is provided with a liquid outlet aperture 21 which is disposed at a distance of not more than one half an inch from the adjacent surface 4 of the strip 2.

Extending axially throughout the length of the tubular portion 20 is a wire electrode 22 which carries a high voltage and which extends through the outlet aperture 21, the passivating liquid applicator 16 being so sized that the tubular portion 20 and wire electrode 22 collectively resemble a hypodermic needle. The wire electrode 22 and the strip 2 are electrically connected (by means not shown) in a circuit such that a high intensity electric discharge is created between the wire electrode 22 and the zinc coated surface 4 so as to produce an electrostatic field in which the passivating solution is atomised.

The passivating liquid applicator 16 is provided at its upper end with an enlarged diameter portion 23 which is supplied (by means not shown) with air which is at atmospheric pressure and which is thus at a pressure exceeding that prevailing externally of the passivating liquid applicator 16. Due to the pressure differential between the pressure of this air and the pressure prevailing in the vacuum chamber 1, the solution in the passivating liquid applicator 16 is forced through the outlet aperture 21 and through the said electrostatic field so as to be deposited on the zinc surface 4. As will be appreciated, the amount of passivating solution applied to the surface 4 can be varied and controlled by controlling the said pressure differential, e.g. by altering the pressure in the enlarged diameter portion 23.

The electrostatic field atomises the passivating solution so that the average diameter of the droplets of passivating solution reaching the surface 4 does not exceed 200 microns, the outlet aperture 21 being so disposed that the droplets take a period not exceeding 0.05 seconds to reach the surface 4. The presence of the diethylene glycol in the passivating solution ensures that the latter does not freeze during this period and thus contacts the zinc coating in the liquid state, this being necessary for the solution to effect its passivating function. Although at least some of the droplets may well reach a temperature below their freezing point prior to contacting the strip 2, the small period of time for the travel of the droplets from the outlet aperture 21 to the strip 2 is such that the droplets will merely be subcooled, i.e. will fall to a temperature below their freezing point, but will still remain in the liquid state. For example, the final temperature of the droplets on reaching the strip 2 may be of the order of -70C and may nevertheless remain in the liquid state, notwithstanding the fact that their freezing point may be of the order of 47C.

It is also believed that the high intensity electrical discharge has the effect of causing dissociation of the passivating solution so as to cause the latter to become very reactive.

The use of the hypodermic needle-like passivating liquid applicator l6 enables very small amounts of the passivating solution to be applied to the surface 4. This is extremely important since, for example, if the passivating solution were to be applied to the strip by roller coating, it would be very difficult to apply a thin liquid film thereto, and great expertise on the part of the operator would be required to control the coating thickness. The control of the coating thickness is, however, very important when applying a passivating film, since the latter tends to be friable and, if too much is applied, subsequent paint adhesion can be poor.

In the said third horizontal run, the surface 4 which has been both zinc coated and passivated is lowermost while the surface 3 is uppermost. The surface 3, after its zinc coating has solidified, passes beneath a passivating liquid applicator 24 whose construction is the same as that of the passivating liquid applicator 16. Thus after passing beneath the latter, both of the opposite surfaces 3, 4 of the strip 2 are provided with passivated zinc coatings.

The strip 2- then passes sequentially through a paint applicator 25 and an electron beam curing apparatus 26. In the paint applicator 25 the strip passes between rollers 30, 31 which apply paint to the passivated zinc coatings 12, 13. The paint applied is a radiation curable, non-solvent based paint such as an alkyd, epoxy, or polyester paint which is applied in the form of a hot melt. In the electron beam curing apparatus 26 this paint is cured by electron bombardment.

After leaving the electron beam cun'ng apparatus 26, the strip passes over the rollers 11, 12 and so passes out of the vacuum chamber 1 through the exit seal 6.

Although the invention has been described above primarily with reference to the passivation of a zinc surface, the invention is also applicable to the passivation of surfaces of aluminium, copper, tin plate, steel, alloys such as aluminium/zinc alloys, and substrates coated with metallic oxides such as alumina or silica. Accordingly, the term metallic as used in this specification is to be understood to include both alloys and compounds of metals.

Moreover, although the vacuum coating technique is described above with reference to a zinc coating, the vacuum coating technique may also be used for the coating of large numbers of metals, e.g. aluminium, titanium, copper, or any alloys thereof.

I claim:

1. A method of passivating a metallic surface to improve the subsequent adhesion of paint thereto comprising atomizing an aqueous passivating solution and increasing its reactivity by passing it through an electrostatic field located within a vacuum chamber and directing the atmoized solution onto a metallic surface in said vacuum chamber, the passivating solution containing a low freezing point liquid which ensures that the droplets of the passivating solution are in the liquid state when they contact and react with the metallic surface.

2. A method as claimed in claim 1 in which the passivating liquid is directed through a liquid outlet aperture of a container for the passivating liquid, the container being provided with an electrode which extends through said aperture and about which the passivating liquid flows, and a high intensity electric discharge is created between the said electrode and the metallic surface.

3. A method as claimed in claim 2 in which the passivating liquid is directed through the said outlet aperture by subjecting the passivating liquid in the container to a gas pressure which exceeds that prevailing externally of the container.

4. A method as claimed in claim 1 in which said passivating liquid is a chromating liquid.

5. A method as claimed in claim 1 in which the said metallic surface is that of a zinc coating on a ferrous substrate.

6. A method as claimed in claim 1 in which a metal substrate is provided with a metallic coating, which constitutes the said metallic surface, while the substrate is disposed within the vacuum chamber, molten coating metal being evaporated under the reduced pressure in the vacuum chamber, and the vapour of the coating metal being condensed onto the substrate.

7. A method as claimed in claim 6 in which the metal substrate is a steel strip which is moved continuously through the vacuum chamber, the coating metal being zinc.

8. A method as claimed in claim 1 in which the passivating solution contains at least 20 percent by volume of water.

9. A method as claimed in claim 1 in which the low freezing point liquid is a glycol.

10. A method as claimed in claim 1 in which the pressure in the vacuum chamber does not exceed 10 Torr.

11. A method as claimed in claim 1 in which the passivating liquid reaches the metallic coating in the form of droplets whose average diameter does not exceed 200 microns.

12. A method as claimed in claim l in which, while the substrate is disposed in the vacuum chamber, the passivated surface is first painted with a radiation curable, non solvent based paint, and the paint is then radiation cured.

13. A method as claimed in claim 13 in which the paint is applied to the passivated surface by roller coating. 

2. A method as claimed in claim 1 in which the passivating liquid is directed through a liquid outlet aperture of a container for the passivating liquid, the container being provided with an electrode which extends through said aperture and about which the passivating liquid flows, and a high intensity electric discharge is created between the said electrode and the metallic surface.
 3. A method as claimed in claim 2 in which the passivating liquid is directed through the said outlet aperture by subjecting the passivating liquid in the container to a gas pressure which exceeds that prevailing externally of the container.
 4. A method as claimed in claim 1 in which said passivating liquid is a chromating liquid.
 5. A method as claimed in claim 1 in which the said metallic surface is that of a zinc coating on a ferrous substrate.
 6. A method as claimed in claim 1 in which a metal substrate is provided with a metallic coating, which constitutes the said metallic surface, while the substrate is disposed within the vacuum chamber, molten coating metal being evaporated under the reduced pressure in the vacuum chamber, and the vapour of the coating metal being condensed onto the substrate.
 7. A method as claimed in claim 6 in which the metal substrate is a steel strip which is moved continuously through the vacuum chamber, the coating metal being zinc.
 8. A method as claimed in claim 1 in which the passivating solution contains at least 20 percent by volume of water.
 9. A method as claimed in claim 1 in which the low freeziNg point liquid is a glycol.
 10. A method as claimed in claim 1 in which the pressure in the vacuum chamber does not exceed 10 4 Torr.
 11. A method as claimed in claim 1 in which the passivating liquid reaches the metallic coating in the form of droplets whose average diameter does not exceed 200 microns.
 12. A method as claimed in claim 1 in which, while the substrate is disposed in the vacuum chamber, the passivated surface is first painted with a radiation curable, non solvent based paint, and the paint is then radiation cured.
 13. A method as claimed in claim 13 in which the paint is applied to the passivated surface by roller coating. 